It is increasingly common in the oil and gas industry to use hydraulic fracturing (colloquially known as “fraccing” or “fracking”) to aid in the recovery of petroleum fluids such as crude oil and natural gas from subsurface formations. Hydraulic fracturing is a process involving the injection of a “fraccing fluid” under pressure into spaces such as cracks and fissures within a subsurface petroleum-beating formation, such that the fluid pressure forces the cracks and fissures to become larger, and/or induces new fractures in the formation materials, resulting in more and/or larger flow paths through which petroleum fluids can flow out of the formation and into a well drilled into the formation. Fraccing fluids typically carry particulate materials called “proppants” that are intended to stay inside the enlarged or newly-created subterranean fissures after the fraccing fluid has been drained out of the formation and hydraulic pressure has been relieved.
There are various different types and formulations of fraccing fluids, but regardless of the type of fraccing fluid being used, one thing that is common to all fraccing operations is the need for temporary storage of very large volumes of fraccing fluid, both to provide a reservoir of frac fluid for injection into subsurface formations, and to store frac fluid circulated out of the well after completion of fraccing operations. Storage tanks having volumes of 250,000 to 1,500,000 U.S. gallons or more are commonly required for this purpose. For practical and environmental reasons, such tanks are typically of modular design so that their components can be shipped by truck to remote well sites, where they can be erected on site and eventually disassembled and shipped off site after they are no longer needed. The costs of shipping storage tank components to and from remote well sites and the costs of erecting and disassembling the tanks on site can be considerable. Accordingly, modular storage tank systems that minimize these costs are highly desirable.
Open-top storage tanks most commonly are circular, as this is the most stable and efficient structural configuration for a liquid storage tank. Modular circular tanks typically comprise multiple horizontally-curved steel wall panels having a radius corresponding to the radius (or half-diameter) of the finished tank. The vertical side edges of the curved wall panels abut and are fastened to the vertical edges of adjacent wall panels by suitable structural connection means, such that when all of the wall panels have been erected and interconnected, they form a circular tank having a particular height, diameter, and liquid storage capacity. External braces are typically installed at intervals around the perimeter of the tank to stabilize the panel assembly, and a suitable liquid-tight liner is installed inside the tank, covering a prepared around surface inside the tank perimeter and extending up and typically over the tank wall. The tank is then ready to receive a fraccing fluid or other liquid that needs to be stored.
Field connection of adjacent modular tank wall panels is commonly facilitated by fabricating the panels with continuous steel end plates or structural angles (a.k.a. angle irons) along their vertical side edges, such that the face of each end plate (or the face of one leg of each angle iron) lies in a plane coincident with a radius of the assembled tank, and perpendicular to the tangent line of the immediately adjacent region of the curved panel. Therefore, the faces of the end plates on adjacent panel edges will come into mating contact upon erection, such that the panels can be securely connected using structural bolts extending through bolt holes provided in the panels' vertical end plates or angle irons.
These field connections between adjacent tank wall panels must carry tensile forces induced by hoop stresses in the walls of the completed tank due to hydrostatic forces exerted by the liquid stored in the tank. The magnitude of the hoop stresses in the tank wall is proportionate to the density of the stored liquid, and it increases linearly with the depth of liquid in the tank. Accordingly, the tensile force that needs to be transferred across the vertical joints between adjacent tank wall panels, per unit of vertical distance, will increase linearly toward the bottom of the tank. The most efficient and economical structural design will therefore result in the bolt spacing in the panel end plates (or angle irons) being increasingly closer toward the bottom of the tank wall.
Alternatively, the bolt hole spacing could conceivably be kept constant by using different sizes of structural bolts at different locations. This alternative would require stocks of different sizes of bolts and would give rise to the risk that bolts that are too small might inadvertently be used in lower regions of the tank, potentially leading to catastrophic failure of the panel connections. However, it could be workable subject to appropriate quality control and field inspection during tank construction.
Modular circular storage tanks as described above are typically designed and fabricated with tank wall panels intended to be tank-diameter-specific. In other words, for a given finished tank diameter, the wall panels will have a radius of curvature corresponding to one-half the tank diameter. Accordingly, in order to accommodate different tank volume requirements, it is necessary to provide multiple sets of tank wall panels having different radii of curvature. This increases the overall cost of maintaining a stock of modular tank assemblies sufficient to meet anticipated requirements.
In addition, modular tanks with tank-diameter-specific wall panels can increase tank assembly transportation costs, such as when a tank of one size is used on one drilling site, and when the drilling rig is later moved to another well site (which might be comparatively close by) at which the frac fluid storage requirements are significantly less than or greater than at the first site. In that scenario, if the storage tank used at the first site has a capacity greater than required at the second site, it could be moved to and used at the second site to save transportation costs (as compared to transporting the tank away from the first site and shipping a smaller tank to the second site). However, that option is disadvantageous in that the larger tank is being inefficiently used, an unnecessarily large area on the well site needs to be prepared to erect the tank, and the tank erection and disassembly costs will be greater than if a smaller tank had been used.
In the alternative scenario where the tank storage capacity at the second site is greater than the requirement at the first site, there will be no alternative but to ship out the tank used at the first site and transport a larger tank to the second site.
For these reasons, there is a need for systems and methods for constructing modular storage tanks in which the modular wall panels can be used to construct tanks of different diameters. The present disclosure is directed to that need.